The present invention relates generally to mass spectrometry and, more particularly, to an apparatus that utilizes the detected position and detected time-of-flight of an electrostatically rastered ion beam to determine the speed and energy-per-charge ratio of individual ions from which the mass-per-charge ratio thereof can be derived. This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy to The Regents of the University of California. The government has certain rights in the invention.
Mass spectrometers are used to measure and analyze the chemical composition of substances. In general, they comprise an ion source, where neutral atoms or molecules from a solid, liquid, or gaseous sample are ionized, a mass analyzer where the atoms or molecules are separated according to their respective masses or their mass-per-charge ratios, and a detector. Various types of mass spectrometers exist, such as, for example, magnetic field spectrometers, quadrupole mass spectrometers, and time-of-flight mass spectrometers.
Magnetic sector mass spectrometers use a magnetic field or combined magnetic and electrostatic fields to measure the ion mass-per-charge. In one type of magnetic sector geometry (see, e.g., A. O. Nier, Rev. Sci. Instr. 18, 398 (1947) and L. Holmlid, Intl. J. Mass Spectrom. and Ion Phys. 17, (1975)), only one mass-per-charge species is detected at any time, so the magnetic field strength, and, if present, the electric field strength are varied to obtain a mass spectrum comprising of multiple mass-per-charge species. A major limitation of this type of sector mass spectrometer is the time that is required to scan the entire mass range one mass at a time.
In another type of magnetic sector mass spectrometer, ions are analyzed and focused onto a position-sensitive detector, resulting in the identification of ion mass-per-charge by spatial dispersion (see, e.g., J. Mattauch and R. Herzog, Zeitschrift fur Physik 89, 786 (1934)). While multiple mass-per-charge species can be detected simultaneously, the spatial resolution of the detector typically limits this type of spectrometer to a narrow mass range.
Quadrupole mass spectrometers utilize a mass filter having dynamic electric fields between four parallel electrodes (see, e.g., Quadrupole Mass Spectrometry and its Applications, ed. Peter H. Dawson (American Institute of Physics, New York, 1995)). These fields are tailored to allow ions having a single mass-per-charge to pass through the filter at a time. One major limitation of quadrupole mass spectrometers is the time required to scan the entire mass range one mass at a time.
Time-of-flight mass spectrometry (TOFMS) can simultaneously detect ions over a wide mass range (see, e.g., M. Guilhaus, J. Mass Spectrom. 30, 1519 (1995)). Mass spectra are derived by measuring the times for ions to traverse a known distance. Generally, an ion""s mass is derived in TOFMS by measurement or knowledge of an ion""s energy E, measurement of the time t1 that an ion passes a fixed point in space P1, and measurement of the later time t2 that the ion passes a second point P2 in space is located a distance d from P1. Using an ion beam having known energy-per-charge E/q, the time-of-flight (TOF) of the ion is equal to t2xe2x88x92t1, and the ion speed is v=d/(t2xe2x88x92t1). Since E=0.5 mv2, the ion mass-per-charge m/q is represented by the following equation:                               m          q                =                                            2              ⁢                                                E                  ⁡                                      (                                                                  t                        2                                            -                                              t                        1                                                              )                                                  2                                                    qd              2                                .                                    (1)            
Implementation of a typical time-of-flight mass spectrometer known in the art is shown in FIG. 1 hereof. A monoenergetic ion bunch, 10, is introduced into a drift region, 12, at time t1, and the time for ions from the front of the bunch to the back of the bunch to pass through the entrance aperture, 14, is xcex94t1. The entire path the ions traverse is evacuated (not shown in FIG. 1). Several types of gating exist to introduce an ion bunch with a small xcex94t1. For example, a pulsed, charged grid may be placed in front of aperture 14 to gate the ions entering drift region 12, a pulsed laser can be utilized to generate bunches of ions, or an ion beam can be periodically deflected onto the entrance aperture using an electric field generated between two conducting plates, 16, as shown in FIG. 1. The ions traverse drift region 12 and disperse in space according to their speed and, therefore, mass, and impinge upon detector, 18: the fastest, lightest ions are detected first, and the slower, heavier ions strike at a later time according to Equ. 1.
Conventional time-of-flight mass spectrometry has several major limitations.
First, the time during which the ion bunch transits the entrance aperture introduces an error, xcex94t1, in the mass-per-charge measurement; therefore, xcex94t1 must be made short compared to the time-of-flight of the ions across the drift tube. Additionally, the lightest ions from a new bunch of ions can be admitted into the drift tube generally only after the heaviest ions from the preceding ion bunch are detected. While sophisticated techniques have been developed to overcome the limitation of overlapped spectra (see, e.g., U.S. Pat. No. 5,396,065 for xe2x80x9cSequencing Ion Packets For Ion Time-Of-Flight Mass Spectrometryxe2x80x9d which issued to C. A. Myerholtz, et al. on Mar. 07, 1995, J. R. D. Copley, Nucl. Instr. and Meth. in Phys. Res. A291, 519 (1990), and G. Wilhelmi et al., Nucl. Instr. and Meth. in Phys. Res. 81, 36 (1970)), conventional time-of-flight mass spectrometers are inefficient since the duty cycle, which can be defined as the fraction of time that ions can enter the drift tube for analysis, is generally much less than unity. Second, it may be observed from Equ. 1 that the mass-per-charge ratio measurement is dependent on the distance d that the ion has traveled over the drift time t2xe2x88x92t1. Therefore, any errors in this time difference will produce corresponding errors in the derived mass-per-charge ratio. These errors are typically minimized by employing a long drift tube and a detector with a small detection area so that d is accurately known.
More recently, a non-time-of-flight technique has been used to determine the mass and velocity of ions. In U.S. Pat. No. 5,726,448 for xe2x80x9cRotating Field Mass and Velocity Analyzerxe2x80x9d which issued to Steven Joel Smith and Ara Chutjian on Mar. 10, 1998, a rotating field mass and velocity analyzer having a cell with four walls, time dependent RF potentials applied to each of the walls and a detector is described. The time-dependent RF potentials create an RF field in the cell which effectively rotates within the cell. An ion beam accelerated into the cell is dispersed by the RF field according to the mass-to-charge ratio and velocity distribution of the ion beam. The ions of the beam either collide with the ion detector or deflect away therefrom depending on the mass-to-charge ratio, the RF amplitude, and the RF frequency. The detector counts the incident ions to determine the mass-to-charge ratio and the velocity distribution in the ion beam. Thus, ions that traverse a dynamic, (time-varying) electric field follow trajectories that are dependent on mass. Since the detector is located close to the cell (no time-of-flight tube), the resolution of the apparatus is limited by the time-of-flight of the ions.
Accordingly, it is an object of the present invention to provide an apparatus having a duty cycle of approximately unity for quantitatively measuring the speed, energy-per-charge, and mass-per-charge of ions.
Another object of the invention is to provide an apparatus for quantitatively measuring the speed, energy-per-charge, and mass-per-charge of ions where the deflected trajectory of an individual ion is independent of its mass.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the apparatus for measuring the mass-per-charge of individual ions in a beam of ions hereof may include: means for generating a collimated, continuous beam of ions travelling along a chosen axis; electrostatic deflection means disposed about the chosen axis such that the beam of ions passes unobstructed therethrough; a voltage source for establishing a chosen pattern of time-varying voltages onto the electrostatic deflection means; and a position-sensitive detector for detecting both the position and time of arrival of ions in the beam of ions having individual masses, said detector being disposed along the axis at a distance such that the beam of ions takes a long time to traverse the distance between the second electrostatic deflection electrodes and the position-sensitive detector when compared with the time the beam of ions takes to traverse both sets of the electrostatic deflection electrodes, whereby the detected position of each of the ions in the group of ions provides information from which the ion mass-per-charge is determined.
Preferably, the means for electrostatic deflection includes pairs of parallel planar electrodes disposed symmetrically about the axis and such that the plane of adjacent planar electrodes are substantially perpendicular.
It is also preferred that the first voltage source generates a first continuous sine wave and the second voltage source generates a second continuous sine wave, whereby the phase of the first sine wave and the phase of the second sine wave are 90xc2x0 out of phase.
In another aspect of the present invention, in accordance with its objects and purposes, the method for determining the mass-per-charge of an ion hereof may include the steps of: generating a collimated, continuous beam of ions travelling along a chosen axis; electrostatically deflecting the ion beam from the chosen axis using a chosen pattern of time-varying voltages; permitting the deflected ions to drift in a zero electrostatic field drift region for a chosen period; and detecting both the position and time of arrival of ions in the beam of ions having individual masses using a position-sensitive detector, wherein the detector is disposed along the axis at a distance such that the drift period of the beam of ions is long when compared with the time the beam of ions spend during the electrostatically deflecting step, whereby the detected position of each of the ions in the group of ions provides information from which the ion mass-per-charge ratio is determined.
Preferably, the step of electrostatically deflecting the ion beam from the chosen axis using a chosen pattern of time-varying voltages is accomplished using a first pair of parallel planar electrodes and a second pair of parallel planar electrodes disposed such that the plane of the first pair of planar electrodes and the plane of the second pair of planar electrodes are substantially perpendicular, a first voltage source for establishing a chosen pattern of time-varying voltages onto the first pair of planar electrodes, and a second voltage source for establishing a chosen pattern of time-varying voltages onto the second pair of planar electrodes.
It is also preferred that the first voltage source generates a first continuous sine wave and the second voltage source generates a second continuous sine wave, whereby the phase of the first sine wave and the phase of the second sine wave are 90xc2x0 out of phase.
Benefits and advantages of the present invention include the ability to reliably acquire and analyze mass spectra with high sensitivity, high accuracy, and high speed (high duty cycle, since ions continuously traverse the present apparatus), without the necessity of ion gating, over a broad range of ion masses. Ion energy can also be determined.